You are what you eat! This popular sentence coined by nutritionists in the last century refers to the simple truth that the food we eat has some bearing on our health and body. More than that, scientists substantiated the link between food and health by showing that a balanced, but restricted, diet can even extend lifespan in a wide variety of organisms. Yet, the benefit of an extended lifespan seems to be at the expense of fecundity as poorly fed organisms produce fewer offspring. The maintenance of vital body functions and reproduction appear to compete for limiting resources. Investigating this phenomenon, a team of British scientists led by Linda Partridge have shown in a recent Nature paper that this view is not entirely correct, as a single amino acid can fully restore the fecundity of long-living fruit flies.
Dietary research over the last few years has shown that rich diets shorten lifespan because of a dietary imbalance rather than excess calories. Moreover, experiments performed in flies and rodents indicated that lifespan and fecundity depend on the specific nutrient content of the diet, and certain amino acids appear to play a pivotal role. Partridge and her team went on to dissect this phenomenon in more detail in the fruit fly Drosophila. Feeding insects chemically defined diets, the scientists identified specific nutrients that affect lifespan and fecundity of the flies. The addition of amino acids shortened lifespan and increased egg laying.
The next logical step was to identify which amino acids account for the effects on lifespan and fecundity. The team found that the addition of 10 essential amino acids increased the insect's fecundity while reducing their lifespan as much as if the insects were fed on a full balanced diet. So the amino acids were responsible for the increased fertility and reduced lifespan, but which ones?
Next the scientists narrowed down the responsible amino acids by feeding the insects on a restricted diet while adding back 9 of the 10 essential amino acids, leaving out methionine. This time they could not increase the insect's fecundity, but when they added methionine alone to the diet of flies on a restricted diet the flies' fecundity was returned to normal. So the addition of methionine is sufficient to increase fecundity.
However, methionine is not the only amino acid that is necessary to reduce life expectancy. Other amino acids in combination with methionine are required to shorten life expectancy because: reintroducing methionine to the restricted diet did not shorten the lifespan; and addition of an amino acid mixture that lacked only methionine did not shorten the insect's life span either. So, in addition to methionine, one or more further essential amino acids are necessary to shorten lifespan.
Having established that methionine is involved in shortening life expectancy, the team wondered how methionine affects lifespan and found that the insulin/insulin-like growth factor signaling (IIS) pathway mediates the effects of amino acids on lifespan and fecundity.
Partridge and her team have clearly demonstrated that the extension of lifespan observed under dietary restriction is due to an amino acid imbalance in the diet – at least in fruit flies. The possibility that even mammals might profit from suitably balanced diets, maintaining the benefits of dietary restriction on longevity without the drawback of reduced fecundity, is very exciting.
